The present invention relates to a composition comprising ammonia and an anti-foaming agent, to a process for substantially suppressing foaming during the vaporization of ammonia, and to a process for using the vaporized ammonia for producing chemicals.
Ammonia can be used in a variety of industrial applications. Typical chemical compounds that can be produced from ammonia include hydrogen cyanide, amines, nitriles and nitric acid.
Hydrogen cyanide can be produced from natural gas, ammonia, and oxygen over a platinum/rhodium gauze catalyst at a temperature greater than 1000xc2x0 C. (the xe2x80x9cAndrussow Processxe2x80x9d). Hydrogen cyanide can also be produced from methane and ammonia passed through porous ceramic tubes lined or coated with platinum, at about 1300xc2x0 C. (the xe2x80x9cBMAxe2x80x9d process). Still further hydrogen cyanide can be produced from propane and ammonia in the presence of an electric current, typically at temperatures greater than 1500xc2x0 C. (the xe2x80x9cShawinigan Processxe2x80x9d).
Nitriles and derivatives can be produced from ammonia and a hydrocarbon, for example, in the catalytic ammoxidation of propylene to form acrylonitrile. Nitriles are important industrial chemicals, especially in the plastics, surface coatings, and adhesive industries. For example, acrylonitrile and methacrylonitrile can be used to produce acrylic fiber, as an intermediate in the syntheses of antioxidants, pharmaceuticals, dyes and surface active agents.
Ammonia is generally supplied to the processes by either direct pipeline or barge transfer. Ammonia supplied to and taken from storage tanks has a high foaming potential, which may be due to a threshold concentration of one or more impurities contained thereinxe2x80x94these impurities can be present either from concentration in the ammonia process or due to impurities received from barge transfers.
Before entering the reactors for producing hydrogen cyanide or a nitrile, for example, the ammonia feed may be flashed or heated to convert it from liquid state to vapor state. The ammonia feed can also be mixed with other feeds and heated. Heating or flashing ammonia to produce ammonia vapor, particularly at low pressures such as, for example, below 100 psig (about 700 kPa), results in foaming situations. This foam can result in various process limitations, such as, for example, low vaporizer capacity, carryover of material to downstream equipment, such as reactors, resulting in catalyst damage and high manufacturing costs, unstable flow control, inappropriate material on the inlet side of relief valves, which is a safety hazard.
It is well known in the ammonia industry that ammonia contains small quantities of oil, mostly occurring from compressor seal leakage. Water is also present in many cases to prevent corrosion. Suggested causes of foam include the oil, detergents present in the oil, water, non-condensable gases such as CO2, N2, H2, and non-volatile materials such as iron. Eliminating the oil via dry seals on compressors on barges and in the ammonia production unit may be considered, but is an expensive option and, since the actual cause for foaming is unknown, dry seals may not offer a real solution.
Therefore, there is an ever-increasing need to develop a process for handling ammonia to produce an ammonia that is substantially free of, or has reduced, foam potential.
According to the first embodiment of the invention a composition is provided, comprising ammonia and an anti-foaming agent wherein the composition is substantially free of water or is substantially anhydrous. This composition is especially effective when using ammonia that has concentrated levels of impurities from an ammonia synthesis process or ammonia that has been retrieved from ammonia storage tanks.
According to a second embodiment of the invention, a process that can be used for vaporizing ammonia is provided. The process comprises contacting an ammonia-containing fluid with a foam-suppressing amount of an anti-foaming agent wherein the fluid is substantially free of water or is substantially anhydrous and subsequently vaporizing the ammonia by known means.
According to a third embodiment of the invention, a process for use of ammonia vapor in a chemical process is provided. The process comprises contacting, in the presence of an ammoxidation catalyst, an oxygen-containing fluid, a hydrocarbon and an ammonia vapor composition.
The term xe2x80x9cfluidxe2x80x9d used herein refers to liquid, gas, or combination thereof. The term xe2x80x9cgasxe2x80x9d includes vapor. The term xe2x80x9csubstantiallyxe2x80x9d denotes xe2x80x9cmore than trivialxe2x80x9d. The term xe2x80x9canti-foaming agentxe2x80x9d refers to a chemical that, when added to a fluid, can substantially reduce the surface activity of the fluid thereby substantially preventing the fluid from foaming. The terms xe2x80x9canti-foaming agentxe2x80x9d and xe2x80x9canti-foamxe2x80x9d can be used interchangeably. The term xe2x80x9csubstantially free of waterxe2x80x9d means less than about 5, preferably less than about 3, and most preferably less than about 1 weight % water, based on the total weight of the composition to which the term refers.
This invention applies to any process where liquid ammonia is being vaporized or heated prior to utilization of the ammonia.
According to the first embodiment of the invention, a fluid composition comprising, consisting essentially of, or consisting of ammonia and an anti-foaming agent is provided.
The ammonia can be any ammonia, i. e., liquid ammonia, gaseous ammonia, or combinations thereof. The ammonia is advantageously ammonia that has concentrated levels of impurities from an ammonia synthesis process, referred to herein as xe2x80x9ccold ammoniaxe2x80x9d or ammonia that has been retrieved from ammonia storage tanks, xe2x80x9cstorage ammoniaxe2x80x9d. Cold ammonia is defined herein to mean ammonia from an ammonia synthesis process wherein the pressure has been let down to near atmospheric in order to provide process cooling and to allow for storage. Storage ammonia is defined herein to mean ammonia comprising cold ammonia from ammonia synthesis as well as added ammonia from barges, tank cars, etc., which is stored in storage tanks at atmospheric pressure and temperature below xe2x88x9230xc2x0 C., at a location accessible to subsequent chemical processing. Warm ammonia herein means ammonia from an ammonia synthesis process wherein the ammonia stream is taken off some intermediate location of the final flash stages such that the resulting pressure of the liquid stream is substantially higher than atmospheric pressure and is generally at ambient temperature conditions. Cold ammonia typically contains impurities from the ammonia process. Storage tank ammonia typically will contain the same impurities as cold ammonia in addition to water, oils, etc. due to contamination from transfers. Cold ammonia and storage ammonia have been found to develop severe foaming problems when vaporized for introducing into subsequent chemical processes.
Any anti-foaming agent that can substantially prevent an ammonia-containing fluid from foaming, especially during vaporization of the fluid, used in the invention. Generally such anti-foaming agents include, but are not limited to, silicones, siloxanes, especially polydimethylsiloxane, hydrocarbon oils such as decane, and combinations thereof. The presently preferred anti-foaming agent is polydimethylsiloxane for it is readily available and effective. For uses wherein the vaporized ammonia is subsequently used in the presence of a platinum-based catalyst, it is preferred that the anti-foaming agent is free of hydrocarbon oils.
Generally, the anti-foaming agent can be present in the composition in a foam-suppressing amount, which is the amount that can substantially reduce the surface activity of ammonia. Such foam-suppressing amount can be in the range of from about 1 to 5000, preferably about 5 to 4000, and most preferably 5 to 3000 mg/kg (ppm) of ammonia. Generally, the concentration of the anti-foaming agent can depend on the ammonia quality and the amount of blowdown taken from the vaporizer. Blowdown is defined herein to mean the amount of material drawn from the bottom of a vaporizer during vaporization.
The composition can be produced by any methods known to one skilled in the art such as, for example, mixing the anti-foaming agent with ammonia. According to the second embodiment of the invention, the process for producing the composition of ammonia and an anti-foaming agent comprises, consists essentially of, or consists of contacting an ammonia-containing fluid with a foam-suppressing amount of an anti-foaming agent wherein the fluid is substantially free of water. Typically the ammonia will have a temperature in the range of about xe2x88x9233xc2x0 C. to about 15xc2x0 C. Cold ammonia and storage ammonia will typically be pre-heated to a temperature of 8xe2x88x9215xc2x0 C. before contacting with the anti-foam. Contemporaneously with, or after, contacting the ammonia with the anti-foam, the resulting mixture is treated under conditions effective to vaporize the ammonia.
Conditions effective to vaporize the ammonia/anti-foam mixture should be sufficient to effect the suppression of foaming during vaporization. Generally, conditions can include heating to a temperature in the range of from about 0 to 30, preferably 0 to about 20 and most preferably 0 to 10xc2x0 C., under a pressure that accommodates the temperature range, and for a period of time in the range from 1 second to about 10 hours.
Upon completion of the vaporizing, the anti-foaming agent can be separated and recovered, by any means known to one skilled in the art, for reuse. The fluid containing ammonia and the anti-foaming agent can also be used as such in a process such as that disclosed hereinbelow. By vaporizing the ammonia/anti-foam composition, such that foaming does not occur advantages are gained in subsequent chemical processes, such as stable flow control during times when cold ammonia or storage tank ammonia is being fed to the process, reduction/elimination of downstream catalyst damage, increasing holding capacity of vessels and improving efficiency of evaporation equipment, thereby increasing productivity in a chemical process, without needing to add costs.
According to the third embodiment of the invention, a process is provided herein for producing a product wherein the process comprises use of ammonia vapor that has been treated with an anti-foam prior to vaporization. In one example, a process comprises, consists essentially of, or consist of contacting, in the presence of an ammoxidation catalyst, an oxygen-containing fluid, a hydrocarbon and an ammonia vapor composition to produce a nitrile. Said ammonia vapor composition is prepared by vaporizing a composition comprising ammonia and an anti-foaming agent as described above.
According to the invention, the term xe2x80x9cnitrilexe2x80x9d refers to a compound having the formula of RCN in which R is hydrogen or an organic radical having 1 to about 10 carbon atoms per radical. The presently preferred nitrites are acrylonitrile, hydrogen cyanide, methacrylonitrile, and derivatives thereof, or combinations thereof. The hydrocarbon can be methane, ethylene, propylene, propane, isobutylene, butene, pentene, 2-methyl-butene, 2-methyl-pentene, hexene, or combinations of two or more thereof.
The contacting of a hydrocarbon with ammonia and an oxygen-containing fluid is generally carried out in the gas phase in a suitable vessel such as, for example, a fluidized bed reactor having an air compressor and a quench column. A hydrocarbon such as, for example, propylene and the ammonia/anti-foam composition can be vaporized and introduced into the vessel or reactor.
The molar ratio of hydrocarbon to ammonia can be any ratio so long as a nitrile can be produced. Generally, the molar ratio can be in the range of from about 0.1:1 to about 10:1, preferably about 0.2:1 to about 5:1, and most preferably about 0.5:1 to about 2:1. The contacting can be carried out under any suitable condition such as a temperature in the range of from about 250 to about 600, preferably about 300 to about 550, and most preferably about 350 to about 500xc2x0 C., under a pressure that can accommodate the temperature range, and for a time sufficient to produce a nitrile, generally about 0.01 second to about 2 hours.
Ammoxidation catalysts are well-known in the art and typically multi-component mixed metal oxides, especially based on bismuth-molybdenum oxides. The preferred catalyst is a typical Bi/Mo-based catalyst supported on an inorganic support such as, for example, silica. The contacting produces a product mixture comprising a nitrile, if R does not equal H, and hydrogen cyanide, generally in gas or vapor phase.
The product mixture can be further purified by any means known to one skilled in the art such as that disclosed in U.S. Pat. No. 3,936,360. Because the purification is well known, the description of which is omitted herein.
Any oxygen-containing fluid that comprises about 1 to about 100 weight % oxygen can be used. Air is a preferred fluid. The most preferred is compressed air.
In a second example, a process comprises, consists essentially of, or consist of contacting, in the presence of platinum-based catalyst, an oxygen-containing fluid, and an ammonia vapor composition to produce nitric acid. The ammonia vapor composition is prepared by vaporizing a composition comprising ammonia and an anti-foaming agent as described above.
In a third example, a process comprises, consists essentially of, or consist of contacting, a hydrocarbon, and an ammonia vapor composition at a temperature greater than 1000xc2x0 C. to produce hydrogen cyanide. The ammonia vapor composition is prepared by vaporizing a composition comprising ammonia and an anti-foaming agent as described above. In a first embodiment, the hydrocarbon is preferably natural gas and the process is performed in the presence of a platinum/rhodium gauze catalyst and oxygen. In a second embodiment, the hydrocarbon is methane and the process in performed in the presence of platinum coated porous ceramic tubes. In a third embodiment, the hydrocarbon is propane and the process is performed in the presence of an electric current.
The following examples are intended to illustrate the invention and are not to be construed to unduly limit scope of the invention.